1. Field of the Invention
The present invention relates to diversity reception devices suitable for use in on-vehicle receivers for orthogonal frequency division multiplex (OFDM) terrestrial digital broadcasts.
2. Description of the Related Art
FIG. 2 illustrates the configuration of a typical known diversity reception device in which the carrier-to-noise (C/N) ratio of a diversity-synthesized signal is maximized. In FIG. 2, one reception system has an antenna 21, a receiver 22 connected to the antenna 21, a variable gain amplifier 23 connected to the output terminal of the receiver 22, and a detector 24 for detecting the level of a signal received by the receiver 22 so as to control the gain of the variable gain amplifier 23. The other reception system has an antenna 25, a receiver 26 connected to the antenna 25, a variable gain amplifier 27 connected to the output terminal of the receiver 26, and a detector 28 for detecting the level of a signal received by the receiver 26 so as to control the gain of the variable gain amplifier 27.
A received signal output from one variable gain amplifier 23 and a received signal output from the other variable gain amplifier 27 are combined by an adder 30. A phase shifter 31 is disposed between the gain amplifier 27 and the adder 30. The phase difference between a received signal output from the variable gain amplifier 23 and a received signal output from the other variable gain amplifier 27 is detected by a phase detector 32, and the phase shifter 31 is controlled in accordance with the detected phase difference.
With this configuration, the gain of the variable gain amplifier 23 is set in accordance with the level detected by the detector 24. Similarly, the gain of the variable gain amplifier 27 is set in accordance with the level detected by the detector 28. The received signal output from the variable gain amplifier 27 is controlled by the phase shifter 31 so that it becomes in phase with the received signal output from the variable gain amplifier 23. Accordingly, the C/N ratio of the signal synthesized by the adder 30 is maximized (see, for example, Digital Musen Tsushin-no Henfukucho (Modulation/Demodulation in Digital Wireless Communication) edited by The Institute of Electronics, Information and Communication Engineers (IEICE), Feb. 10, 1996, pp. 189-191 and FIG. 5.19).
The received signal synthesized by the adder 30 is converted into a baseband signal by an OFDM demodulator (not shown).
In the diversity reception device configured as described above, since each variable gain amplifier 23 or 27 is feedforward-controlled in the corresponding reception system, a change in the gain of the variable gain amplifier 23 or 27 due to, for example, the ambient temperature or the power supply voltage, appears at the output side as it is. Accordingly, the level of each signal input into the adder 30 becomes different from the level set in proportion to the input level of the variable gain amplifier 23 or 27. As a result, the C/N ratio of the synthesized signal output from the adder 30 is not maximized.